Hydraulic machines with variable displacement volume or swallowing capacity comprise a hydrostatic displacement unit which, for instance, sets the angle position of a swash plate or bent axis. This displacement unit comprises two essential elements for adjusting and controlling the displacement volume of the hydraulic machine. That is firstly the control unit, which converts incoming mechanical, pneumatical, hydraulical or electrical control signals into adequate control volume flow rates for the second essential element, the servo displacement unit, which engages with a displacement element of the hydraulic machine. The control unit and the servo displacement unit are connected to each other via fluid conducting control lines, which supply, respectively discharge the volume flow rates necessary for the servo displacement unit. In order to set a certain displacement volume of the hydraulic machine against actions of internal spring forces and external operational forces, the control volume flow rates have to be supplied under adequate pressure. Such a displacement unit for hydraulic machines is disclosed, for instance, in DE 10 2004 033 376 B3.
The control signals for the control unit are converted by actuators, preferably in axial force actions on the control spool. The signals can be of various manners, for instance, mechanically, hydraulic-mechanically and as well as electrically. For the conversion of electric signals solenoids or switching magnets serves as actuators. Often the control unit is of a mechanical design with movable parts, which, for instance, are implemented as control valves comprising a control cylinder and a control spool designed to move longitudinally. The control spool usually is moved by actuators engaging with the same in axial direction. Naturally, friction is acting during the movement of these parts, which leads to a mechanical Hysteresis. Such a Hysteresis shows among others that equal control signals on the control unit causes different control volume flow rates, respectively different control pressures, depending on whether the control signal is set with an increasing signal ramp or with a decreasing signal ramp. This is due to the fact that the motion of the parts of the control unit show different directions according to the increasing or the decreasing control signal ramp. Because of this, the friction forces act in different directions and, mostly, also with different strengths.
In general, hysteresis are not wanted as these influence in a hard controllable manner the control volume flow rate to be adjusted by the control unit according to given control signals, such that no unique value can be associated to one single control signal for the displacement of a hydraulic machine, as it depends on the control ramp with which the control signal was set, respectively out of which position the control spool was displaced in the control cylinder.
For example, in control units with electric actuation of the actuator, the hysteresis of actual actuator force, respectively of the actuator position can be a counteracted in that the electrical control signal is superimposed by an oscillation signal. This leads to a vibration of the movable parts of the actuator and therewith to a permanent high frequent reversal of the friction forces, which, for instance, are superimposed to the steady direction of forces resulting from the control signal ramp. Herewith the influence of the static friction on the actuator itself as well as on the actuated parts of the control unit, respectively on the control spool, is minimized, however, eventually, inaccuracies occur in the control of the control volume flow rates and, hence, of the displacement volume of the hydraulic machine if the control spool is to be displaced with pulsing force. When applying electric control signals, dither or pulses width modulated (PWM) signals are used, whose amplitude and frequency have to be adjusted to the requirements of a concrete design of the displacement unit. Pulse width modulated signals show the disadvantages vantages that their amplitudes depend on the height of the electrical signals and, hence, are not optimal in each and every control state. Dither signals are capable to hold the amplitudes in a broad but finally also limited band in a constant and optimal manner independent from the height of the electrical signal. However, not every amplitude, which, eventually, is optimal for minimizing the Hysteresis, is also suitable for the electronic control. Furthermore, it is difficult to apply a Hysteresis-reducing oscillation signal, if the activation is non-electric, respectively hydromechanic or pneumatic-mechanic.
In JPS62218676 (A) means for the reduction of hysteresis at a control spool are described, with which pulsations in the hydraulic fluid are acting directly on a front face of a control spool via an amplifying chamber arranged externally of a control unit and without interposition of a further actuator, whose pressure serves as control signal for the control unit. The pulsations or pressure fluctuations are created in the amplifying chamber by means of a mass oscillating on a spring. This system is sophisticated as the amplifying chamber needs its own charge pump, and, furthermore, this results in increasing space requirements for the additional assembly groups.